Electronic switches, such as cross bar switches, often consist of a single NMOS transistor connected between an input port and an output port. When the switch is open, there is a high impedance path between the input and the output to provide circuit isolation. When the switch is closed, it provides a low resistance path from the input to the output. These switches allow for signal propagation in either direction. Therefore, either port could be an input or an output port.
As these switches are utilized to propagate signals at higher and higher frequencies, the switches must have a lower RC network value to pass the higher frequency signals and a small resistance delta across the input voltage in order to minimize signal distortion.
One known solution to the high frequency data switching problem is to use a charge pump to increase the voltage on the gate of the pass transistor. This requires an on-chip oscillator and produces a higher current drain than is desired. Another known solution to this problem is to bias the back gate of the pass transistor to either the source or the drain input voltage in order to make VBS equal to zero volts. This biasing technique causes the NMOS threshold voltage to lower and the drive strength to increase, which reduces the on resistance. However, this circuit arrangement can compromise the isolation of the output from the input if the voltage applied to the input terminal goes far enough below ground to turn on the parasitic NPN transistor or cause the pass transistor to conduct because the VGS voltage increases. If the output is high, this can cause the output to be pulled down which can cause a glitch in the output. If the glitch is of sufficient magnitude, it can cause the output to change state.
Accordingly, there is a need for a switch circuit which has a reduced on resistance and provides undershoot protection.